I made my own staging using pressure treated timber from Wickes, 2 years ago; it cost me about £43 to make an 8' length with lower shelf also. If you are interested I could post a pic. Took me one afternoon to make.

Took the words right out of my mouth Duncan and homebase or similar stores do a light wood book case, which when stained with the stuff you can paint fences with are ideal for putting trays of seedlings on, at a squeeze you can fit three trays on each shelf. Can't remember the cost but they are less than £30 may even by less than £20.

If you get the taller book case it can be sawn in half and the shelves then fit on your potting bench .

Mattbeer, I've just had a peek at your profile. Lucky you, with your garden.

You have certainly come to tghe right place for genuine help and advice. You ask.....We answer.

6x4 is a bit of a problem, however. I would suggest, if you just want bench style stagging. Utilise just one side of the house. Work inside the G/H. Basically four timbers say waist high. These will be the legs. Two timbers for the length. Four corner struts. Two lower bars to keep the legs rigid. The top can be slatted or a sheet of external ply. Nail each corner to the top of the legs and runners. Cover with plastic sheet. At the blank end of the house, you can easily construct a framework with removeable shelves. This will act as your seed tray area etc.

If as I understand. You are going to build your own greenhouse. Perhaps part of the bench can be incorporated into the main structure.

B+Q sell one for about 40 pounds, self assembly but instructions poor, got one for Christmas - cobbled it together, works quite well! Had a good laugh working it out and it's slightly different to the picture provided! Look in the pound shops for cheap shelving. Lidl and Aldi sell decent tools and odds and ends at various intervals and seeds